Nuclear Energy: Fission, Fusion and the Future
Climate change sounds like an existential crisis over the course of this century. The planet is hurtling towards rising average temperatures and there is increasing demand to shift towards low-carbon energy sources.
Energy drives economies and sustains civilizations. Energy powers cellular functions at the microscopic level to the sun that powers life on Earth at the macroscopic level. Early humans discovered fire, a source of energy that they used for warmth, light and cooking food. Cooking food enabled us to take larger calories in a shorter amount of time. More food enabled our brains, energy-intensive machines, to grow larger. This also fundamentally changed how humans spent time and energy. Before cooking, humans spent large portions of the day hunting, foraging and eating. But with fire and cooking, humans started to spend more time thinking, socializing and creating new tools.
200 years ago, humans discovered fossil fuels that could be used as energy and thus began the industrial revolution. It led to more productive ways of living and enabled progress as we have never seen before. Access to cheap energy at a large scale came as both boon and bane.
The boon: A single steam engine powered by coal could do the work of a dozen horses or 5 dozen men in much less time. Rapid progress in low-cost automobiles and widespread use of electricity changed the way society consumes energy. Energy demand started to double every 10 years and then gradually rise at an exponential rate. Our World in data claims "It(Energy) has allowed work to become more productive, and people in industrialized countries are much richer than their ancestors, work much less, and enjoy much better living conditions than ever before. Energy access is therefore one of the fundamental driving forces of development."
The bane: Energy from fossil fuels has led to an increase in air pollution, deaths during mining or excavation of resources and greenhouse gases. Energy (electricity, heat and transport) is responsible for 73.2% of GHG emissions.
Given that development and improved standards of living are in direct correlation with energy access, the demand for energy will only continue to increase further as we pull more and more people out of poverty. It is clear that there is a demand for a transition to an energy source that is clean and safe across the globe.
(Even if one were a climate change denier, it is inevitable that there will come a time when non-renewable resources run out, and there will be a need to transition off of fossil fuels long before then, to have a smooth transition and not dampen the economic progress.)
Nuclear and renewable sources of energy are the way forward. While renewable sources are the most sought solution, there are places in the world owing to poor atmospheric or climate conditions that are not suitable for renewable sources of energy. Eg: South East Asia is prone to heavy and variable monsoon that affect prospects of renewable sources of energy. Let's save renewable sources of energy for another day and dive into nuclear today.
Our world in Data claims Nuclear Energy is the safest based on the number of deaths per TWh produced. However, there is more to it. Is the technology safe enough that even if the core explodes, can the knock-off effects be limited?
But we need to understand the two kinds of nuclear sources of energy:
Nuclear Fission: The splitting of a large atomic nucleus, such as Uranium or Thorium, into smaller nuclei and releasing energy in the process.
NulcearFusion: Energy is produced by smashing together light atoms. In other words, the opposite of Fission.
Nuclear energy has been touted as an alternative to fossil fuels. Fission reactors can be expensive, generate large amounts of radioactive waste and come with safety risks. All the fiascos and failures in the past on the nuclear energy front have been caused by nuclear fission reactors. The impact of the Fukushima reactor which led to countries like Germany shutting down their nuclear plants is debatable. Although the reactor is directly accountable for 1 death, subsequent opposition to nuclear energy has caused more damage to the planet.
Nuclear Fusion has more advantages than Nuclear fission in that it is cleaner, cheaper and safer, there has been no technologically viable nuclear fusion reactor that has been built. However, there has been tremendous progress over the last decade in both nuclear fission and nuclear fusion technologies that are commercially as well as technologically viable while being extremely safe. Let’s explore them a little bit.
Nuclear Fission:
Nuclear Fission is taking the heavy elements and splitting them up into lighter elements while creating energy during the process.
A general overview of a Fission reactor is as follows:
Fissionable isotopes like Uranium 235, 233 or Plutonium 239 absorb a neutron and then split up into fission products. In that process, they release heat as well as eject more neutrons to continue the chain reaction. A moderator is usually employed to slow down the neutrons. The heat released is passed to a working fluid, either gas or water, which then runs a steam turbine to generate electricity. In terms of fuel, there is at least 3 times more thorium than uranium on Earth. And it's waste largely decays in 100s of years rather than 10,000s. There are different designs for a fission reactor:
Molten Salt Reactor (MSR):
MSRs operate on the same basic principle as conventional nuclear power reactors — controlled fission to produce steam that powers electricity-generating turbines. The primary fuel is a molten salt mixture - a salt that is solid at standard temperature and pressure but enters the (ionic) liquid phase due to elevated temperature. The liquid salt can be both the fuel that produces heat and the coolant that transports the heat to the power plant.
Credit: Ensuring Nuclear Performance
The salt is heated above the melting point so that it becomes fluid. One of the common designs for a Molten Salt Reactor is to dissolve fissile material in the molten salt. The fuel flows around graphite rods which moderate the energy of the neutrons. Other designs include liquid or solid fuels contained in rods but with molten salt as coolant. There is a broad range of fuel and salt compositions while some designs do not require a moderator at all.
Replacing water as coolant and operating at atmospheric pressure removes the possibility of explosion risks. In the case of MSR, Nuclear reactions are easier to control because liquid salts expand. In the event of an unexpected rise in temperature, this expansion shuts down the reaction. In the event of failsafe such as power failure, a freeze plug is in place to dump the fuel into tanks and stop the reaction.
MSRs do not produce dangerous and radioactive fission gases that are under pressure, as they are naturally absorbed into the molten salt. This removes the possibility of contaminating large areas of land and making the process safe.
Since they operate at a higher temperature compared to conventional reactors, they can generate electricity more efficiently. The use of liquid fuel allows for real-time waste processing. Additionally, the technology allows for new fuel to be introduced during operation removing the need for shutting down the reactor for refuelling.
There are two major challenges with MSR
Understanding and mitigating the corrosion of structural materials, and
Development of reliable and efficient chemical separations
Seaborg is a Danish startup with 25 Million $ in funding that develops the Compact Molten Salt Reactor based on molten fluoride salt. Flibe Energy is a startup with 4.2 Million $ based in the US that designs liquid-fluoride thorium reactor (LFTR), a molten-salt reactor. Moltex is a UK-Canada stable salt reactor based nuclear fission company that became the first nuclear energy startup to raise funds through crowdfunding.
TWR is another type of fission process that converts fertile material - material that is not fissile but can be converted to fuel by removing neutron i.e nuclear transmutation - in addition to fissile material such as uranium. TWR is considered efficient because it can use depleted fuel without reprocessing. The technology is claimed to be cheaper, safer and non-proliferating which enables it to be made available to all nations.
Bill Gates-funded Terrapower is developing TWR but seem to be far-fectched from realizing their goal anytime soon.
Small Modular Reactors(SMR):
SMR reactors, as the name describes are smaller than conventional reactors(take up ~1% of the space) and modular in design. The design permits the reactor to generate 50x more power compared to a typical reactor. Each modular reactor is designed with passive safety features to avoid a meltdown while eliminating the use of pumps and pipes that may fail or cause an accident.
Credit: sciencemag.org Oklo is an SMR startup with 25 Million $ funding that became the first advanced fission company in the US to have a license application accepted by the U.S. Nuclear Regulatory Commission. X-Energy is another US startup with 6 Million $ in funding that is designing the safest, most efficient and most advanced small modular reactors.
Fission reactors are expensive to build, although they already account for all the nuclear power supply across the globe. Globally, around 10% of electricity comes from nuclear fission and countries like France(70%) and Sweden(40%) depend already on it.
Fusion Energy:
Nuclear Fusion powers the solar system through the sun. It involves ‘fusing’ two hydrogen atoms that combine to form Helium while releasing energy in the process. It is the fusion of elements that create other elements that make up everything in this universe - from planets to humans. Hydrogen and its isotopes - deuterium and tritium - are virtually free, limitless, and equally distributed around the globe.
Credits: Auclimate
What startups and labs are trying to recreate is the core of the sun inside a lab. Heat a bunch of hydrogen atoms at 10s of millions of degrees inside an oven-like-setup that is well insulated so that these atoms fuse together to create huge amounts of energy. It is claimed that in such nuclear reactions, the power generated per fuel input is 200 Million times more compared to the same fuel required to burn hydrocarbons. Why is that so?
There are two fundamental forces that play a role here:
Strong nuclear force holds protons and neutrons together inside the nucleus, and
Coloumb force which causes positively charged protons to repel each other
And these forces are opposing in nature. The strong nuclear force is more powerful than Coulomb forces for atoms with small nuclei and the difference in force is released as energy in form of emission of subatomic particles or radiation, sometimes both. For elements bigger than Iron, the fusion of atoms do not produce energy since Coulomb force is stronger due to more number of protons.
The forces that dictate nuclear binding are far stronger than forces that hold electrons in orbit around a nucleus( quiet rightly called weak nuclear force). Therefore, chemical reactions that happen when fuels such as hydrocarbons are burnt produce much less energy. This is why fusion fuels offer vastly higher energy density than chemical methods—about a million times denser than fossil fuels.
When confined at these high temperatures, the nuclei can collide with sufficient speed to overcome Coulomb repulsion and fuse together. Besides the temperature, the fusion process should also be maintained at certain pressure and confinement time to create a plasma - a state of matter in which an ionized gaseous substance becomes highly electrically conductive.
To attain plasma, an atom is heated to a point(called ionization energy level), at which its electrons get stripped away leaving behind a bare nucleus called an ion. With multiple atoms going through the same process, a hot cloud of ions and electrons are formed and the hot cloud is plasma. Since plasma, or the cloud of ions and electrons, is electrically conductive and magnetically susceptible it is easier to control these particles.
here are many methods that are under research to infuse plasma. Some of the most advanced concepts in the making are:
Magnetic Confinement Fusion:
Race hot plasma around in a magnetically confined environment with an internal current. Eg: Tokamak uses a torus (donut-shaped) magnetic bottle as the confined environment. Fusion fuel (dueterium and trituium) is heated to form a high-temperature plasma which is then made to fuse by squeezing the plasma.
Credits: ITER
Currently, with an input power of 70 MegaWatts(MW), an output of 500 MW is achieved for a few hundred seconds but the aim is to sustain the fusion reaction for a long period of time. There are lithium blankets placed outside the plasma reaction chamber to absorb the high-energy neutrons released from the fusion reaction that can be used to make more tritium fuel.
The blankets also get heated by the neutrons and heat is transferred by a water-cooling loop to make steam that drives electrical turbines to produce electricity.
Commonwealth Fusion Systems is a young startup, an MIT spin-off, with 200M $ in backing that is developing a revolutionary superconducting magnet technology to accelerate the path to commercial fusion energy. They hope to have a commercial plant ready by 2025.
Tokamak Energy, with a funding of 50 Million $, is a UK startup that aims to deliver clean and abundant fusion power by 2030. The company's chief Scientist Mikhail Gryaznevich is a leading authority in the field of Tokamak designs.
While Tokamaks create magnetic fields using an internal current, Stellarators attempt to create a natural twisted plasma path, using external magnets. Renaissance Fusion, a French startup is the first company to focus on Stellarator technology.
TAE technologies is the oldest, well established and most funded (600 Million $) in the fusion energy spectrum. Unlike a tokamak, their design generates a magnetic field through eddy currents - current induced within conductors by a changing magnetic field - in the plasma that reverses the axial field.
Inertial Confinement Fusion:
Initiate nuclear fusion reactions by heating and compressing pellets(~10mg) or fuel targets that usually contains a mixture of deuterium and tritium. Usually, high energy beams of laser or beam of ions are delivered to the outer layer of the target to compress and heat the fuel.
Credits: Lawrence Livermore National Laboratory
As the heated outer layer explodes, a reaction force accelerates the inner layer inwards compressing the target which creates shockwaves further compressing the target. This heats the fuel at the center creating a fusion reaction. During the final implosion, the fuel core reaches 100 times the density and ignites at 100 Million deg C. Currently, the technology can produce only the same output for a given input of electricity.
A major challenge with this technology is successfully removing heat from the reaction chamber without interfering with the targets and driver beams.
First Light Fusion and Marvel Fusion are two early-stage European companies that approach ICF with a copper disk at high velocity and laser-based approach, respectively to target pellets.
3. Magnetic Target Fusion:
A hybrid between Magnetic Confinement Fusion and Inertial Confinement Fusion, the system has 3 main components:
a plasma injector, which supplies the fuel i.e converts the hydrogen fuel into a hot plasma and trap it in a magnetic field, ready for compression
an array of pistons, to compress the fuel. To form a perfectly symmetrical shockwave, these pistons must strike within fractions of a second of each other.
and a chamber of spinning liquid metal to hold the fuel and capture the energy.
Making a plasma that is stable is the biggest challenge with this technology. General Fusion is a Canadian company with 200M $ in funding that is most established in the nuclear energy space. As of 2017, the company claimed to have a prototype ready by 2022.
The current challenges with this technology are:
successful removal of heat from the reaction chamber without interfering with the targets and driver beams.
the huge number of neutrons released in the fusion reactions causing the plant to become intensely radioactive as well as mechanically weakening metals.
Conventional metals such as steel used to build fusion plants would have to be replaced frequently owing to their short lifetimes.
If these challenges could be overcome, Nuclear energy could become the holy grail of energy that is abundant, clean, safe while enabling progress both economically and technologically. It seems likely that nuclear energy becomes the norm before the end of the next decade and at least one of the above-mentioned startups could lead the charge.
Resources:
What is Nuclear is a great website run by a group of nuclear scientists that explain the concepts behind nuclear energy and share some interesting information.
Isabelle is a badass social media influencer who shares educational and entertaining content about nuclear energy.
this is a good read, something that I have been thinking about myself lately.